Contact and adsorbent granules

ABSTRACT

The present invention relates to a filtering unit at least partially filled with particles agglomerated from fine-particle iron oxide and/or iron oxyhydroxide by producing an aqueous suspension of fine-particle iron oxides and/or iron oxyhydroxides having a BET surface area of 50 to 500 m 2 /g, and removing the water and dissolved constituents by a set of washing drying and filtering steps and processes of using the particles.

This application is a divisional of U.S. patent application Ser. No.09/962,935, filed on Sep. 25, 2001, now abandoned, entitled CONTACT ANDADSORBENT GRANULES, the contents of which are hereby incorporated byreference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to particles, pellets or granules offine-particle or nanoparticle iron oxides and/or iron oxyhydroxideshaving a large specific surface area (50 to 500 m²/g according to BET),and processes for their production. These pellets have high mechanicalresistance and can be used as a contact, adsorbent, or catalyst for thecatalysis of chemical reactions, for the treatment of fluid media likeliquids and/or for gas, specifically the removal of impurities.

Contact and adsorbent granules, including those based on iron oxidesand/or iron oxyhydroxides, have already been described. They arepredominantly used in continuous processes. They are conventionallyfound in tower or column-type units through which the medium to betreated flows, and the chemical or physical reaction or adsorptionprocesses take place at the outer and inner surface of the granules.Powdered materials cannot be used for this purpose because they compactin the direction of flow of the medium, thereby increasing the flowresistance until the unit becomes blocked. If a unit is cleaned byback-flushing (see below), large amounts of the powder are dischargedand lost or cause an unacceptable contamination of the waste water.

The flowing media also exert forces on the granules, however, which canlead to abrasion and/or movement through to violent agitation of thegranules. Consequently the granules collide, leading to undesirableabrasion. This results in loss of contact or adsorbent material andcontamination of the medium to be treated.

Adsorbents/catalysts containing iron oxides and hydroxides canadvantageously be used e.g. in the area of water purification or gaspurification. In water purification this agent is used in horizontal- orvertical-flow filters or adsorber columns or added to the water to betreated in order to remove dissolved, suspended or emulsified organic orinorganic phosphorus, arsenic, antimony, sulfur, selenium, tellurium,beryllium, cyano and heavy metal compounds from, for example, drinkingwater, process water, industrial and municipal waste water, mineral,holy and medicinal water as well as garden pond and agricultural water.It can also be used in so-called reactive walls to separate the citedcontaminants from ground water and seepage water aquifers fromcontaminated sites (waste disposal sites).

In gas purification the agent is used in adsorbers for bindingundesirable components such as hydrogen sulfide, mercaptans and hydrogencyanide, as well as other phosphorus, arsenic, antimony, sulfur,selenium, tellurium, cyano and heavy metal compounds in waste gases.Gases such as HF, HCl, H₂S, SO_(x), NO_(x) can also be adsorbed.

The removal of phosphorus, arsenic, antimony, selenium, tellurium, cyanoand heavy metal compounds from waste oils and other contaminated organicsolvents is also possible.

Contact and adsorbent granules based on iron oxides and/or ironoxyhydroxides are also used for the catalysis of chemical reactions inthe gas phase or in the liquid phase.

Various methods of removing trace constituents and contaminants fromaqueous systems with the aid of adsorbents are also known.

For example, DE-A 3 120 891 describes a process in which a filtration isperformed using activated alumina with a grain size of 1 to 3 mm for theseparation principally of phosphates from surface water.

DE-A 3 800 873 describes an adsorbent based on porous materials such ase.g. hydrophobed chalk with a fine to medium grain size to removecontaminants from water.

DE-A 3 703 169 discloses a process for the production of a granulatedfilter medium to treat natural water. The adsorbent is produced bygranulating an aqueous suspension of kaolin with addition of powdereddolomite in a fluidised bed. The granules are then baked at 900 to 950°C.

A process for the production and use of highly reactive reagents forwaste gas and waste water purification is known from DE-A 40 34 417.Mixtures consisting of Ca(OH)₂ with additions of clays, stone dust,entrained dust and fly ashes, made porous and having a surface area ofapprox. 200 m²/g, are described here.

The cited processes have the disadvantage that the component responsiblein each case for the selective adsorption of constituents of the mediato be cleaned, in other words the actual adsorbent, must be supplementedwith large quantities of additives to enable it to be shaped intogranules. This significantly reduces the binding capacity for the watercontaminants to be removed. Moreover, subsequent reprocessing or reuseof the material is problematic since the binder substances first have tobe separated out.

DE-A 4 214 487 describes a process and a reactor for the removal ofimpurities from water. The medium flows horizontally through afunnel-shaped reactor, in which finely divided iron hydroxide inflocculent form is used as a sorption agent for water impurities. Thedisadvantage of this process lies in the use of the iron hydroxide inflocculent form, which means that because there is little difference indensity between water and iron hydroxide, a reactor of this type can beoperated at only very low flow rates and there is a risk of the sorptionagent, which is possibly already loaded with contaminants, beingdischarged from the reactor along with the water.

JP-A 55 132 633 describes granulated red mud, a by-product of aluminiumproduction, as an adsorbent for arsenic. This consists of Fe₂O₃, Al₂O₃and SiO₂. No mention is made of the stability of the granules or of thegranulation process. A further disadvantage of this adsorbent is thelack of consistency in the composition of the product, its unreliableavailability and the possible contamination of the drinking water withaluminium. Since aluminium is suspected of encouraging the developmentof Alzheimer's Disease, contamination with this substance in particularis to be avoided.

DE-A 19 826 186 describes a process for the production of an adsorbentcontaining iron hydroxide. An aqueous polymer dispersion is incorporatedinto iron hydroxide in water-dispersible form. This mixture is theneither dried until it reaches a solid state and the solid material thencomminuted mechanically to the desired shape and/or size or the mixtureis shaped, optionally after a preliminary drying stage, and a finaldrying stage then performed, during which a solid state is achieved. Inthis way a material is obtained in which the iron hydroxide is firmlyembedded in the polymer and which is said to display a high bindingcapacity for the contaminants conventionally contained in waste watersor waste gases.

The disadvantage of this process lies in the use of organic binders,which further contaminate the water to be treated due to leaching and/orabrasion of organic substances. Furthermore, the stability of theadsorbent composite is not guaranteed in extended use. Bacteria andother microorganisms can also serve as a nutrient medium for an organicbinder, presenting a risk that microorganisms may populate the contactand thereby contaminate the medium.

The presence of foreign auxiliary substances, which are required for themanufacture of the adsorbents, during reprocessing, recycling or reuseof used adsorbents is disadvantageous in principle because the reuse ofpure substances is less problematic than is the case with mixedsubstances. For example, polymeric binders are disadvantageous when ironoxide-based adsorption materials are reused as pigments for concretecoloration because these binders inhibit dispersion of the pigment inliquid concrete.

DE-A 4 320 003 describes a process for the removal of dissolved arsenicfrom ground water with the aid of colloidal or granulated ironhydroxide. Where fine, suspended iron(III) hydroxide products are used,it is recommended here that the iron hydroxide suspension be placed infixed-bed filters filled with granular material or other supports havinga high external or internal porosity. This process likewise has thedisadvantage that, relative to the adsorbent “substrate+iron hydroxide”,only low specific loading capacities are achievable. Furthermore, thereis only a weak bond between substrate and iron hydroxide, which meansthat there is a risk of iron hydroxide or iron arsenate being dischargedduring subsequent treatment with arsenic-containing water. Thispublication also cites the use of granulated iron hydroxide as anadsorption material for a fixed-bed reactor. The granulated ironhydroxide is produced by freeze conditioning (freeze drying) of ironhydroxide obtained by neutralisation of acid iron(III) salt solutions attemperatures of below minus 5° C. This production process is extremelyenergy-intensive and leads to heavily salt-contaminated waste waters.Moreover, as a result of this production process only very smallgranules with low mechanical resistance are obtained. When used in afixed-bed reactor, this means that the size spectrum is significantlyreduced by mechanical abrasion of the particles during operation, whichin turn results in finely dispersed particles of contaminated oruncontaminated adsorption agent being discharged from the reactor. Afurther disadvantage of these granules lies in the fact that theadsorption capacity in respect of arsenic compounds is reducedconsiderably if the granules lose water, by being stored dry forextended periods for example.

Adsorbent/binder systems obtained by removing a sufficiently largeamount of water from a mixture of (a) a crosslinkable binder consistingof colloidal metal or non-metal oxides, (b) oxidic adsorbents such asmetal oxides and (c) an acid such that components (a) and (b) crosslinkto form an adsorbent/binder system, are known from U.S. Pat. No.5,948,726. According to the disclosure, colloidal alumina or aluminiumoxide is used as binder.

The disadvantage of these compositions lies in the need to use acid intheir production (column 9, line 4) and in the fact that they are notpure but heterogeneous substances, which is undesirable both for theproduction, regeneration, removal and permanent disposal of suchadsorbents, e.g. on a waste disposal site. The scope of disclosure ofthis publication is also intended to include adsorbents that aresuitable for the adsorption of arsenic; specific examples are notprovided, however. Aluminium oxide is known to be significantly inferiorto iron oxides in regard to force of adsorption for arsenic.

Continuous adsorbers, which are commonly grouped together in parallelfor operation, are preferably used for water treatment. To free drinkingwater from organic impurities, for example, such adsorbers are filledwith activated carbon. At peak consumption times, the availableadsorbers are then operated in parallel to prevent the flow rate fromrising above the upper limit permitted by the particular arrangement. Attimes of lower water consumption, individual adsorbers are taken out ofoperation and can be serviced, for example, whereby the adsorptionmaterial is subjected to special loads, as described in greater detailbelow.

The use of granules, which can be produced by compacting e.g. powderediron oxide using high linear forces, has also already been considered.Such granules have already been described as a means of homogeneouslycolouring liquid concrete. The use of high linear forces for compactingis extremely expensive and energy-intensive, and the stability of thecompacted materials is inadequate for extended use in adsorbers.

The use of such materials in adsorbers, for example, particularlycontinuous models, for water purification is therefore of only limitedinterest. During maintenance or cleaning of adsorber plants byback-flushing in particular (see below), such granules lose largeamounts of substance due to the associated agitation. The abradedmaterial renders the waste water from back-flushing extremely turbid.This is unacceptable for a number of reasons: firstly, adsorptionmaterial, which is heavily laden with impurities and therefore toxicafter extended use, is lost. Secondly, the stream of waste water isladen with abraded material, which can sediment, damaging piping systemsand ultimately subjecting the waste treatment plant to undesirablephysical and toxicological stresses, to name but a few reasons.Preferably the abrasion should be below 20% by weight, more preferablybelow 15% by weight, 10% by weight or most preferably below 5% by weightaccording to the method described in the examples of the presentinvention.

An object of the present invention was therefore to provide a contact oran adsorbent/catalyst based on iron-oxygen compounds in pellet form,exhibiting high mechanical resistance in conjunction with a good bindingcapacity for contaminants contained in liquids and gases without theneed to use organic binders or inorganic foreign binders to achieveadequate mechanical resistance, and plants operated with such media.This object is achieved by the contacts or adsorbents/catalystsaccording to the invention, their preparation, their use and the unitsfilled therewith.

SUMMARY OF THE INVENTION

The invention relates to a unit suitable for the through-flow of a fluidmedium at least partially filled with particles agglomerated fromfine-particle iron oxide and/or iron oxyhydroxide, wherein thefine-particle iron oxide and/or iron oxyhydroxide displays a particlesize of up to 500 nm and a BET surface area of 50 to 500 m²/g.

The invention also relates to a process for the production of particlesfrom fine-particle iron oxide and/or iron oxyhydroxide comprising thesteps of producing an aqueous suspension of fine-particle iron oxidesand/or iron oxyhydroxides having a BET surface area of 50 to 500 m²/g,and removing the water and dissolved constituents by either I) a) firstremoving only the water from the suspension, b) introducing the residuethus obtained in water, c) filtering the material obtained, d) washingthe residue, and e) either e1) completely dehydrating the filter cakeobtained as residue and comminuting the material thus obtained to thedesired shape and/or size or e2) partially dehydrating the filtercake toobtain a paste, shaping the paste and subsequently additionally dryingthe paste until a pellet is obtained, or II) a) filtering thesuspension, b) washing the residue, c) either c1) completely dehydratingthe filter cake obtained as residue in the form of a solid to semisolidpaste and then comminuting the material thus obtained to the desiredshape and/or size or c2) partially dehydrating the filtercake to obtaina paste, shaping the paste, and subsequently additionally drying thepaste until a pellet is obtained.

DETAILED DESCRIPTION OF THE INVENTION

To prepare the granules according to the invention, an aqueoussuspension of fine-particle iron oxyhydroxides and/or iron oxides isfirst produced according to the prior art. The water and constituentsdissolved within it can be removed from this in two different ways:

Method 1:

For applications in which lower demands are made of the mechanicalstrength of the granules/contacts, only the water is removed initially,e.g. by evaporation. A residue is obtained which in addition to thefine-particle iron oxide and/or hydroxide also contains the entire saltcontent. This residue is redispersed in water after being dried, forwhich purpose only relatively little shear force needs to be applied.This suspension is then filtered and the residue washed until it issubstantially free from salts. The filter cake obtained as residue is asolid to semisolid paste which generally has a water content of between10 and 90 wt. %.

This can then be completely or partially dehydrated, and the materialthus obtained can then be comminuted to the desired shape and/or size.Alternatively the paste or filter cake, optionally after predrying toachieve a sufficiently solid state, can undergo shaping followed by(additional) drying until a pellet state is achieved. The subsequentapplication of the granules determines the preferred procedure to befollowed for their production, which can be determined by the personskilled in the art in the particular field of application by means ofsimple preliminary orienting experiments. Both the directly dried filtercake and the dried shaped bodies can then be used as contact oradsorbent.

Method 2:

For applications in which higher demands are made of the mechanicalstrength of the granules/contacts, the suspension is filtered and theresidue washed until it is substantially free from salts. The filtercake obtained as residue is a solid to semisolid paste. This can then becompletely or partially dehydrated, and the material thus obtained canthen be comminuted to the desired shape and/or size. Alternatively thepaste or filter cake, optionally after predrying to achieve asufficiently solid state, can undergo shaping followed by (additional)drying until a pellet state is achieved. The subsequent application ofthe granules determines the preferred procedure to be followed for theirproduction, which can be determined by the person skilled in the art inthe particular field of application by means of simple preliminaryorienting experiments. Both the directly dried filter cake and the driedshaped bodies can then be used as contact or adsorbent.

Although the products obtained according to method 1 are lessmechanically resistant, filtration can be performed more easily andquickly. The fine-particle pigments isolated in this way can moreover beincorporated very easily into paints and polymers, for example, becauseconsiderably less shear force is required than is needed to incorporatethe fine-particle pigments obtained according to method 2.

The fine-particle iron oxide and/or iron oxyhydroxide used has aparticle size of up to 500 nm, preferably up to 100 nm, particularlypreferably 4 to 50 nm, and a BET surface area of 50 to 500 m²/g,preferably 80 to 200 m²/g.

The primary particle size was determined by measurement from scanningelectron micrographs, e.g. at a magnification of 60000:1(instrument:XL30 ESEM FEG, Philips). If the primary particles areneedle-shaped, as in the α-FeOOH phase for example, the needle width canbe given as a measurement for the particle size. Needle widths of up to100 nm, but mainly between 4 and 50 nm, are observed in the case ofnanoparticle α-FeOOH particles. α-FeOOH primary particles conventionallyhave a length:width ratio of 5:1 to 50:1, typically of 5:1 to 20:1. Thelength:width ratio of the needle shapes can be varied, however, bydoping or by special reaction processes. If the primary particles areisometric, as in the α-Fe₂O₃, γ-Fe₂O₃, Fe₃O₄ phases for example, theparticle diameters can quite easily also be below 20 nm.

By mixing nanoparticle iron oxides or iron (oxy)hydroxides with pigmentsand/or Fe(OH)₃, the presence of the cited pigment or nucleus particlesin their known particle morphology, held or glued together by thenanoparticle nucleus particles or the amorphous Fe(OH)₃ polymer, can bedetected on the scanning electron micrographs.

Products obtainable by methods 1) and 2) can then be comminuted further,for example by rough grinding or grinding. However, since the productsreduce in size on first coming into contact with water, for example whena freshly charged adsorber unit is first filled with water, this willgenerally be unnecessary.

Granulation of a semi-wet paste has proven effective as another methodof producing granules. Here pellets or strands are formed from asemi-wet paste, e.g. using a simple perforated metal sheet, a roll pressor an extruder, and either dried immediately or additionally shaped intoa spherical or granular form by means of a spheroniser. The still wetspherules or granules can subsequently be dried to any moisture contentwhatsoever. A residual moisture content of <50% is recommended toprevent the granules from agglomerating. A spherical shape of this typecan be advantageous for use in fixed-bed adsorbers due to the improvedpacking in the adsorber vessel that is obtained in comparison withrough-ground granules or pellets in strand form.

The filtration performance of the suspensions can generally be improvedby the use of conventional filtration-improving measures, such as aredescribed for example in Solid-Liquid Filtration and SeparationTechnology, A. Rushton, A. S. Ward, R. G. Holdich, 2nd edition 2000,Wiley-VCH, Weinheim, and in Handbuch der IndustriellenFest/Flüssig-Filtration, H. Gasper, D. Öchsle, E. Pongratz, 2nd edition2000, Wiley-VCH Weinheim. Coagulants can thus be added to thesuspensions, for. example.

Iron carbonates can also be used in addition to or in place of the ironoxyhydroxides.

The products according to the invention can undergo drying in air,and/or in vacuo, and/or in a drying oven and/or on belt dryers or byspray drying, preferably at temperatures from −25 to 250° C.,particularly preferably at 60 to 120° C.

The products according to the invention preferably have a residual watercontent of less than 20 wt. %.

It was found that the pellets or granules obtained in this way have ahigh binding capacity for contaminants contained in water, liquids orgases and they additionally have an adequately high resistance toflowing media in terms of mechanical or hydraulic stressing.

It is particularly surprising that during drying, fine-particle ironoxyhydroxides or iron oxides having large specific surface areassolidify into very hard agglomerates, which without the addition ofbinders have a high mechanical abrasion resistance and high hydraulicresistance to contact with flowing water, and which have a high bindingcapacity for the contaminants and trace constituents contained in thewater.

Transparent iron oxyhydroxide pigments, for example, having an averageparticle size of less than 0.1 μm and specific surface areas of greaterthan 80 m², are suitable for the use according to the invention offine-particle iron oxyhydroxides. Correspondingly fine-particle ironoxide pigments, preferably haematite, magnetite or maghemite, can alsobe used, however.

The production of yellow fine-particle iron oxyhydroxide pigments (e.g.goethite) in the acid or alkaline pH range, known as acid or alkalinenuclei, is known. The production of other fine-particle iron oxide oriron oxyhydroxide pigments is also known. Such pigments can containstructures based on α, β, γ, δ, δ′, ε phases and/or Fe(OH)₂ and mixedand intermediate phases thereof. Fine-particle yellow iron oxyhydroxidescan be calcined to fine-particle red iron oxides.

The production of transparent iron oxides and iron oxyhydroxides isknown e.g. according to DE-A 2 603 050 from BIOS 1144, p. 29 to 33 orfrom FIAT 814, p. 1 to 26.

Fine-particle yellow iron oxyhydroxide pigments are generallysynthesized by precipitating iron(II) hydroxides or carbonates fromcorresponding iron(II) salt solutions such as e.g. FeSO₄, FeCl₂ in pureform or as pickling solutions in the acid or alkaline pH range, followedby oxidation to iron(III) oxyhydroxides (see inter alia G. Buxbaum,Industrial Inorganic Pigments, VCH Weinheim, 2nd edition, 1998, p.231ff). Oxidation of the divalent to the trivalent iron is preferablyperformed with air, whereby intensive aeration is advantageous.Oxidation with H₂O₂ also leads to fine-particle iron oxyhydroxides. Thetemperature chosen for precipitation and oxidation should be as low aspossible in order to obtain very fine-particle yellow pigments. It ispreferably between 15° C. and 45° C. NaOH is preferably used as alkalineprecipitant. Other precipitants, such as KOH, Na₂CO₃, K₂CO₃, CaO,Ca(OH)₂, CaCO₃, NH₃, NH₄OH, MgO and/or MgCO₃, can also be used, however.

By choosing suitable precipitation and oxidation conditions,nanoparticle α, β, γ, δ phases and mixed phases of iron oxyhydroxidesdisplaying a large specific surface area can be prepared, such that thenanoparticles agglomerate in the dry state and possess a high resistanceto mechanical and fluid-mechanical abrasion in comminuted form.

Production of fine-particle iron oxyhydroxides by simultaneous rapidtreatment of iron(II) salt solutions with NaOH and air has proven to beparticularly beneficial in practice because this production method leadsto particularly fine-particle iron (oxy)hydroxides and hence to agreater stability of the finished product in addition to a higher forceof adsorption.

To steer the precipitated pigments in the direction of the extremelyfine-particle character that is required, the precipitations, e.g. ofyellow α-FeOOH as described in U.S. Pat. No. 2,558,303 and U.S. Pat. No.2,558,304, are performed in the alkaline pH range with alkali carbonatesas precipitants, and modifiers such as SiO₂, zinc, aluminium ormagnesium salts, hydroxycarbonic acids, phosphates and metaphosphatesare generally added. Products produced in this way are described in U.S.Pat. No. 2,558,302. Such nucleus modifiers do not interfere with thesubsequent reprocessing, recycling or any other use of the adsorbentsaccording to the invention. In the case of precipitation processes in anaqueous medium, it is known that precipitations in an alkalineenvironment lead to less solidly agglomerated powders than those in anacid environment.

One of the advantages of nucleus modifiers, however, is that an adequatefine-particle character can be obtained even at elevated reactiontemperatures.

DE-A 4 235 945 reports on the production of fine-particle iron oxidesusing a precipitation method in the acid pH range and without modifiers.

DE-A 4 434 669 describes a process by which highly transparent yellow,chemically pure iron oxide pigments can be produced by secondarytreatment thereof with sodium hydroxide solution.

DE-A 4 434 972 reports on highly transparent, yellow iron oxide pigmentsin the α-FeOOH modification having a specific surface area of over 100m²/g and high temperature resistance.

DE-A 4 434 973 describes highly transparent yellow iron oxide pigments,which are produced by means of the process steps of nuclearprecipitation in the acid pH range, nuclear oxidation, nuclearmaturation and pigment formulation.

Red, transparent iron oxide pigments obtained by calcining from yellow,transparent iron oxide pigments are known from DE-A 4 434 668 and DE-A 4235 946.

By preparing diverse phases of iron oxyhydroxides in pure form or in anymixture from iron(II) salt solutions using the known precipitation andoxidation reactions, separating the resultant iron oxyhydroxides out ofthe suspension, optionally after a secondary treatment, by filtering thesalt solution and washing them until they are largely free from salts,preferably down to a residual conductivity of <5 mS/cm, then drying thesolid or semisolid filter cake just as it is or optionally aftermechanical shaping until it achieves a solid state, a mechanicallyhighly resistant material displaying a high binding capacity for thecontaminants conventionally contained in waste waters or waste gases isobtained.

Drying is conveniently performed at temperatures of up to 250° C. Thematerial can also be vacuum or freeze dried. The particle size of thematerial can be freely selected but is preferably between 0.2 and 40 mm,particularly preferably between 0.2 and 20 mm. This can be achieved byshaping the semisolid, pasty filter cake mechanically, before drying, ina granulation or pelletising plant or in an extruder to form shapedbodies whose size is in the range between 0.2 and 20 mm, with subsequentdrying in the air, on a belt dryer or in a drying oven, and/or bymechanical comminution to the desired particle size after drying.

The products described, the process for their production and their userepresent an improvement over the prior art. In contrast to those basedon coarse-particle iron oxyhydroxides and/or oxides, the granulesaccording to the invention based on fine-particle iron oxyhydroxidesand/or oxides can be subjected to much higher stresses and thereforedisplay a much greater abrasion resistance to mechanical and hydraulicstressing. They can be used directly as such. When used in adsorberplants for water purification, for example, there is no need even forcomminution or rough grinding of the crude dry substance initiallyobtained from filter cakes or extruders, since the coarse pellets breakdown independently on contact with water. This results in a randomparticle-size distribution, but no particles of such a size that theyare discharged from the adsorber to any significant extent by theflowing medium.

There is absolutely no need for a separate granulation process, such aswould be necessary when using conventional iron oxyhydroxides in theform of (flowable) powders, either with the aid of foreign binders orusing extremely high linear forces during compacting.

According to the invention, the suspensions of fine-particle ironoxyhydroxides or iron oxides can also be supplemented with conventionalpowdered iron oxyhydroxides or iron oxides. The quantities in each caseare determined by the properties of these powdered iron oxyhydroxides oriron oxides and by the requirements of the product according to theinvention in terms of its mechanical stability and abrasion resistance.Although the addition of powdered pigments will generally reduce themechanical strength of the products according to the invention,filtration of the fine-particle suspensions is made easier. The personskilled in the art and practising in the particular field of applicationwill be able to determine the optimum mixing ratio for the intendedapplication by means of a few orienting experiments.

The nanoparticle nuclei are conveniently produced in an excess of sodiumhydroxide solution.

A quantity of Fe₂(SO₄)₃ corresponding to the NaOH excess can also beadded to the suspensions of the alkaline fine-particle nuclei. Thismeasure considerably improves the filterability of the suspension. Theinitially amorphous Fe(OH)₃ produced matures over time, to the α-FeOOHphase, for example. This ensures that the sodium hydroxide solution usedin excess to produce the alkaline nucleus is completely used up. Thematerial thus obtained also displays large specific surface areas. Justlike the iron oxyhydroxides described above, the material is extremelysuitable for use in adsorbers since it possesses a high resistance tomechanical loading in addition to a high adsorption capacity.

The granules according to the invention are particularly preferably usedin the cleaning of liquids, especially for the removal of heavy metals.A preferred application in this industrial field is the decontaminationof water, particularly of drinking water. Particular attention hasrecently been paid to the removal of arsenic from drinking water. Thegranules according to the invention are extremely suitable for thispurpose, since levels that not only meet but actually fall below eventhe lowest limiting values set by the US authority the EPA can beachieved using the granules according to the invention.

To this end the granules can be used in conventional adsorber units,such as are already used with a charge of activated carbon, for example,to remove other types of contaminants. Batchwise operation, in cisternsor similar containers for example, optionally fitted with agitators, isalso possible. However, use in continuous plants such as continuous-flowadsorbers is preferred.

Since untreated water to be processed into drinking water conventionallyalso contains organic impurities such as algae and similar organisms,the surface of adsorbents, especially the outer surface of granularadsorbents, becomes coated during use with mostly slimy deposits, whichimpede or even prevent the inflow of water and hence the adsorption ofconstituents to be removed. For this reason adsorber units areperiodically back-flushed with water, a process which is preferablyperformed at times of low water consumption (see above) on individualunits that have been taken out of service. The adsorbent is whirled upand the associated mechanical stress to which the surface is subjectedcauses the undesirable coating to be removed and discharged against thedirection of flow during active operation. The wash water isconventionally sent to a sewage treatment plant. The adsorbentsaccording to the invention have proven to be particularly effective inthis process, since their high strength enables them to be cleanedquickly without suffering any significant losses of adsorption materialand without the back-flush water sent for waste treatment being rich indischarged adsorption material, which is possibly already highlycontaminated with heavy metals.

Since the granules according to the invention are free from foreignbinders, the material is comparatively easy to dispose of after use. Forinstance, the adsorbed arsenic can be removed by thermal or chemicalmeans in special units, for example, resulting in an iron oxide pigmentas a pure substance which can either be recycled for use in the sameapplication or supplied for conventional pigment applications. Dependingon the application and legal regulations, the content of the adsorbercan also be used without prior removal of the heavy metals, for exampleas a pigment for colouring durable construction materials such asconcrete, since the heavy metals removed from the drinking water arepermanently immobilised in this way and taken out of the hydrologicalcycle.

The invention therefore also provides water treatment plants orwaterworks in which units filled with the granules according to theinvention are operated, and processes for the decontamination of waterby means of such units, as well as such units themselves.

For many applications, particularly those in which a maximum mechanicalstrength is not required of the granules, the addition of powderedpigments during production of the granules according to the invention isa preferred embodiment.

Thus, for example, up to 40 wt. % of commercial goethite (Bayferrox 920,Bayer AG, Leverkusen DE) can be added to a nucleus suspension accordingto example 2 of the present application if the granules obtainedaccording to the invention are to be used for the removal of arsenicfrom drinking water in adsorbers with a through-flow of water.

The BET specific surface area of the products according to the inventionis determined by the carrier gas process (He:N₂=90:10) using thesingle-point method, according to DIN 66131 (1993). The sample is bakedfor 1 h at 140° C. in a stream of dry nitrogen before measurement.

In order to measure the adsorption of arsenic(III) and arsenic(V), 3liters of an aqueous solution of NaAsO₂ or Na₂HAsO₄, each with thespecified concentration of approx. 2-3 mg/l arsenic, are treated with 3g of the sample to be tested in a 5 liter PE flask for a specific periodand the flask moved on rotating rollers. The adsorption rate of As ionson iron hydroxide over this specific period, e.g. one hour, is stated asmg(As^(3+/5+))/g(FeOOH)·h, calculated from the balance of the As^(3+/5+)ions remaining in solution.

The adsorption of Sb³⁺, Sb⁵⁺, Hg²⁺, Pb²⁺, Cr⁶⁺ or Cd²⁺ ions is measuredin the same way, whereby the desired concentrations are established bydissolving appropriate amounts of Sb₂O₃, KSb(OH)₆, PbCl₂, NaCrO₄ orCdCl₂ in H₂O and adjusting the pH value to 7-9.

The As, Sb, Cd, Cr, Hg or Pb contents of the contaminated ironoxyhydroxide or of the solutions are determined using mass spectrometry(ICP-MS) according to DIN 38406-29 (1999) or by optical emissionspectroscopy (ICP-OES) according to EN-ISO 11885 (1998), withinductively coupled plasma as excitation agent in each case.

The mechanical and hydraulic abrasion resistance was assessed using thefollowing method: 150 ml of demineralised water were added to 10 g ofthe granules to be tested, having particle sizes >0.1 mm, in a 500 mlErlenmeyer flask, which was rotated on a LabShaker shaking machine(Kühner model from Braun) for a period of 30 minutes at 250 rpm.The >0.1 mm fraction was then isolated from the suspension using ascreen, dried and weighed. The weight ratio between the amount weighedout and the amount weighed in determines the abrasion value in %.

The invention is described in greater detail in the following by meansof examples. The examples are intended to illustrate the process and donot constitute a limitation.

EXAMPLES Example 1

237 l of an aqueous iron sulfate solution with a concentration of 150g/l FeSO₄ were prepared at 24° C. 113 l of an aqueous NaOH solution (227g/l) were then quickly added and the light blue suspension then oxidisedwith 40 l of air per hour and per mol of iron for 1.5 hours.

The yellow suspension thus obtained was filtered out through a filterpress and the solid washed until the residual filtrate conductivity was1 mS/cm. The filter cake was in the form of a spreadable and kneadablepaste, which was dried on metal sheets in a circulating air drying ovenat 75° C. until the residual moisture content was 3 wt. %. The driedmaterial was then roughly ground to produce particle sizes of between0.5 and 2 mm. The hard pellets thus obtained were then placed directlyin an adsorber tank.

The product consisted of 100% α-FeOOH with an extremely short-needledhabit, whereby the needles were congregated to form solid macroscopicagglomerates. Using a scanning electron micrograph e.g. at amagnification of 60000:1, the needle widths were measured at between 15and 35 nm, the needle lengths between 150 and 350 nm. The needles wereextremely agglomerated.

The BET specific surface area was 122 m²/g. The adsorption rate forNaAsO₂ with an original concentration of 2.3 mg (As³⁺)/l was 2.14mg(As³⁺)/g(FeOOH)·h, for Na₂HAsO₄ with an original concentration of 2.7mg (As⁵⁺)/l it was 2.29 mg(As⁵⁺)/g(FeOOH)·h.

Example 2

800 l of an aqueous iron sulfate solution with a concentration of 150g/l FeSO₄ were prepared at 29° C. and 147 l of an aqueous NaOH solution(300 g/l) added over 20 minutes with stirring. 2.16 kg of a nucleusmodifier in the form of a 57% aqueous glycolic acid solution were thenadded to the grey-blue suspension formed in order to reduce the particlesize of the nuclei, and oxidation was performed for 7 hours with 38 l ofair per hour and per mol of iron.

The dark brown suspension was filtered out through a filter press andthe solid washed until the residual filtrate conductivity was 1 mS/cm.The filter cake was dried at 70° C. in a circulating air drying oven toa residual moisture of 5%, and the very hard blackish brown dry productwas roughly ground in a roller crusher to particle sizes of up to 2 mm.The fine fraction <0.2 mm was separated out using a screen.

An X-ray diffractogram showed that the product consisted of 100%α-FeOOH. Using a scanning electron micrograph e.g. at a magnification of60000:1, the needle widths were measured at between 15 and 20 nm, theneedle lengths between 50 and 80 nm. The particles were extremelyagglomerated. The BET specific surface area was 202 m²/g. The granulesthus obtained were placed directly in an adsorber tank with no furthertreatment.

The granules displayed an excellent adsorption performance in respect ofthe contaminants contained in the flowing water and demonstrated a highabrasion resistance, particularly when the adsorber tank is beingback-flushed causing the granules to be whirled up strongly. Theabrasion value after 30 minutes was only 1%.

Adsorption performance: The adsorption rate for NaAsO₂ with an originalconcentration of 2.4 mg (As³⁺)/l was 1.0 mg(As³⁺)/g(FeOOH)·h, forNa₂HAsO₄ with an original concentration of 2.8 mg (As⁵⁺)/l it was 2.07mg(As³⁺)/g(FeOOH)·h.

Example 3

1.3 l of an aqueous 300 g/l NaOH solution were added to an α-FeOOHsuspension obtained according to example 2 after a two-hour maturationat 30° C. with stirring, and post-oxidation was performed simultaneouslyfor one hour with 190 l of air. The product was processed as describedin example 2. Fine-particle needles of pure α-FeOOH with a BET specificsurface area of 130 m²/g were obtained. Using a scanning electronmicrograph e.g. at a magnification of 60000:1, the needle widths weremeasured at between 15 and 20 nm, the needle lengths between 50 and 90nm. The needles were extremely agglomerated. The granules proved to bevery mechanically and hydraulically resistant, and the abrasion valuewas only 3.9%.

Adsorption performance: The adsorption rate for NaAsO₂ with an originalconcentration of 2.3 mg/l was 1.1 mg(As³⁺)/g(FeOOH)·h, for Na₂HAsO₄ withan original concentration of 2.8 mg/l it was 1.7 mg(As³⁺)/g(FeOOH)·h.

Example 4

306 l of an aqueous NaOH solution (45 g/l) were prepared at 31° C. and43 l of an aqueous solution of FeCl₂ (344 g/l) quickly added withstirring, and oxidation was then performed with 60 l of air per hour andper mol Fe. The dark yellow suspension thus obtained was processed inthe same way as in example 1.

An X-ray diffractogram showed that the product consisted of 100%α-FeOOH. Using a scanning electron micrograph e.g. at a magnification of60000:1, the needle widths were measured at between 15 and 50 nm, theneedle lengths between 100 and 200 nm. The needles were extremelyagglomerated. The BET specific surface area was 132 m²/g.

The granules thus obtained were placed in an adsorber tank with nofurther treatment. The granules displayed an excellent adsorptionperformance in respect of the contaminants contained in the water anddemonstrated a high abrasion resistance, particularly when the adsorbertank is being back-flushed causing the granules to be whirled upstrongly. The abrasion value after 30 minutes was only 12 wt. %.

Adsorption performance: The adsorption rate for NaAsO₂ with an originalconcentration of 2.4 mg (As³⁺)/l was 2.11 mg(As³⁺)/g(FeOOH)·h, forNa₂HAsO₄ with an original concentration of 2.7 mg (As⁵⁺)/l it was 2.03mg(As⁵⁺)/g(FeOOH)·h.

Example 5

124 l of an aqueous NaOH solution (114 g/l) were prepared at 24° C. and171 l of an aqueous solution of FeSO₄ (100 g/l) quickly added withstirring, and oxidation was then performed with 10 l air per hour andper mol Fe. Immediately upon completion of oxidation, 56 l of an aqueoussolution of Fe₂(SO₄)₃ (100 g/l) were added and stirred for 30 minutes.The yellowish brown suspension thus obtained was processed in the sameway as in example 1.

An X-ray diffractogram showed that the product consisted of 100%α-FeOOH. Using a scanning electron micrograph e.g. at a magnification of60000:1, the needle widths were measured at between 15 and 35 nm, theneedle lengths between 70 and 180 nm. The needles were extremelyagglomerated. The BET specific surface area was 131 m²/g. The abrasionvalue after 30 minutes was only 7 wt. %.

Adsorption performance: The adsorption rate for NaAsO₂ with an originalconcentration of 2.3 mg (As³⁺)/l was 1.7 mg(As³⁺)/g(FeOOH)·h, forNa₂HAsO₄ with an original concentration of 2.7 mg (As⁵⁺)/l it was 1.2mg(As⁵⁺)/g(FeOOH)·h.

Example 6

7905 kg FeSO₄ were prepared, dissolved with water to a volume of 53.3m³, the solution cooled to 14° C. and 1000 kg MgSO₄·7 H₂O added to thissolution. The prepared solution was then diluted at 14° C. with 5056 kgNaOH as a solution with approx. 300 g/l and then oxidised with 4000 m³/hair to a precipitation degree of >99.5%. The batch was washed on afilter press until the residual filtrate conductivity was <1000 μS/cmand the paste pushed through a perforated metal sheet with holediameters of 7 mm, causing it to be formed into strands. The strandswere dried on a belt dryer to a residual moisture of approx. 3%. AnX-ray diffractogram showed that the product consisted of 100% α-FeOOHwith very short needles. Using a scanning electron micrograph e.g. at amagnification of 60000:1, the needle widths were measured at between 30and 50 nm. The needle lengths could not be clearly determined as theneedles were too greatly agglomerated. The BET specific surface area was145 m²/g. The abrasion value after 30 minutes was only 6%.

Adsorption performance: The adsorption rate for NaAsO₂ with an originalconcentration of 2.5 mg (As³⁺)/l was 1.8 mg(As³⁺)/g(FeOOH)·h, forNa₂HAsO₄ with an original concentration of 2.9 mg (As⁵⁺)/l it was 1.5mg(As⁵⁺)/g(FeOOH)·h.

Example 7

4096 kg NaOH (as solution with approx. 300 g/l) were prepared anddiluted with water to 40 m³. 4950 kg FeSO₄ were dissolved with water toform 48.5 m³ solution, cooled to 15° C. and then pumped into theprepared NaOH over 1 h. The suspension was then oxidised with 1500 m³/hair over approx. 2 h. Approx. 2 m³ of the nucleus suspension was washedon a filter press to obtain a filtrate conductivity <1000 μS/cm, thefilter cake was dried in a drying oven at 75° C. and the dried materialroughly ground to particle sizes <1.5 mm. The fine fraction <0.5 mm wasseparated out using a screen. The material thus obtained had a BETspecific surface area of 153 m²/g and consisted of 100% α-FeOOH. Using ascanning electron micrograph e.g. at a magnification of 60000:1, theneedle widths were measured at between 15 and 35 nm, the needle lengthsbetween 50 and 100 nm. The needles were extremely agglomerated.

Adsorption performance: The adsorption rate for NaAsO₂ with an originalconcentration of 2.7 mg (As³⁺)/l was 1.7 mg(As³⁺)/g(FeOOH)·h, forNa₂HAsO₄ with an original concentration of 2.8 mg (As⁵⁺)/l it was 1.4mg(As⁵⁺)/g(FeOOH)·h.

Example 8

3100 kg NaOH (as solution with approx. 100 g/l) were measured out anddiluted with cold water to 31 m³. The temperature of the solution was26° C. 3800 kg FeSO₄ were dissolved with water to form about 38 m³solution, cooled to 13-14° C. and then pumped with stirring into theprepared NaOH. The suspension was then oxidised with 2500 m³/h air inapprox. 75 min. 18.2 m³ FeSO₄ solution (100 g/l) were added at a rate of150 l/min to this suspension with stirring and gassing. The suspensionwas filtered on a filter press and washed until the residual filtrateconductivity was <1000 μS/cm, the paste was pushed through a perforatedmetal plate and were dried on a belt dryer to a residual moisture ofless than 20%. The dry pellets were roughly ground to obtain a particlesize of less than 2 mm. The portion of the particles with less then 0.5mm was removed. The material thus obtained had a BET specific surfacearea of 145 m²/g and consisted of 100% α-FeOOH.

Example 9

An aqueous solution of FeSO₄ (100 g/l) was added at room temperature to1600 g of the alkaline nucleus suspension prepared according to example7 (2.7% FeOOH) with stirring and simultaneous aeration with 130 l/h ofair until a pH of 8 was obtained. The nucleus suspension obtained wasfiltered, washed and the filter cake dried at 75° C. and roughly groundto particle sizes of between 0.5 and 2 mm as described in example 7. Thematerial thus obtained had a BET specific surface area of 163 m²/g andaccording to the X-ray diffractogram consisted of 100% α-FeOOH. Thescanning electron micrograph, e.g. at a magnification of 60000:1, showedthat the needles are extremely agglomerated. Adsorption performance: Theadsorption rate for NaAsO₂ with an original concentration of 2.7 mg(As³⁺)/l was 2.0 mg(As³⁺)/g(FeOOH)·h, for Na₂HAsO₄ with an originalconcentration of 2.7 mg (As⁵⁺)/l it was 1.9 mg(As⁵⁺)/g(FeOOH)·h, forKSb(OH)₆ (original concentration 3.0 mg (Sb⁵⁺)/l) the adsorption was 2.5mg (Sb⁵⁺)/g (FeOOH)·h, for Na₂CrO₄ (original concentration 47 μg(Cr⁶⁺)/l) 42 μg (Cr⁶⁺)/g(FeOOH)·h were adsorbed, for PbCl₂ (originalconcentration 0.94 mg (Pb²⁺)/l) 0.46 mg (Pb²⁺)/g(FeOOH)·h were adsorbed.

Example 10

6.4 l of an aqueous solution of NaOH (100 gA) were prepared at 29° C.with stirring and 12.2 l of an aqueous iron(II) sulfate solution (100g/l) were added with simultaneous introduction of air until a pH of 9was obtained. The suspension thus obtained was processed in the same wayas in example 1. The material had a BET specific surface area of 251m²/g and according to the X-ray diffractogram consisted of 100% α-FeOOH.The scanning electron micrograph shows short, stumpy needles, which areextremely agglomerated. Abrasion performance: 5%.

Adsorption performance: The adsorption rate for NaAsO₂ with an originalconcentration of 2.7 mg (As³⁺)/l was 1.1 mg(As³⁺)/g(FeOOH)·h, forNa₂HAsO₄ with an original concentration of 2.7 mg (As⁵⁺)/l it was 1.0mg(As⁵⁺)/g(FeOOH)·h.

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention except as it may be limited by the claims.

1. Granules comprising agglomerates of fine-particle iron oxyhydroxidein the α-FeOOH phase having a BET surface area of 50 to 500 m²/g andmechanical and hydraulic abrasion resistance of <12%.
 2. The granulesaccording to claim 1, wherein the BET surface area is 80 to 200 m²/g. 3.The granules according to claim 1, wherein the granules have a residualmoisture of <50%.
 4. The granules according to claim 1, wherein thegranules have an arsenic (V) adsorption of >37%.
 5. The granulesaccording to claim 1, wherein the granules have an arsenic (III)adsorption of >40.7%.
 6. The granules according to claim 1, wherein thegranules have a size of from 0.2 to 2 mm.